Malaysia Airlines says it "deeply regrets that we have to assume that MH370 ended in the southern Indian Ocean".

"The ongoing multinational search operation will continue, as we seek answers to the questions which remain," the carrier adds. "Alongside the search for MH370, there is an intensive investigation, which we hope will also provide answers."

Investigators have yet to disclose fully the reasoning behind the conclusion - based on analysis by Inmarsat and UK accident investigators - that MH370 flew to the southern Indian Ocean.

The UK's Air Accidents Investigation Branch says that it has been "using satellite data to determine the area on which to focus the search" in conjunction with Inmarsat, but declines to comment further on the Malaysian-led investigation.

There are also no immediate clues to the circumstances which led the aircraft to stray so far from its course, and no indication as to whether fuel exhaustion or another event finally brought the flight down.